diff --git a/osbase/build/mkimage/osimage/osimage.go b/osbase/build/mkimage/osimage/osimage.go
index 611c2ed..5b17e6b 100644
--- a/osbase/build/mkimage/osimage/osimage.go
+++ b/osbase/build/mkimage/osimage/osimage.go
@@ -130,7 +130,7 @@
 	if err != nil {
 		return nil, fmt.Errorf("failed to open system image: %w", err)
 	}
-	if _, err := io.Copy(blockdev.NewRWS(i.systemPartitionA), systemImage); err != nil {
+	if _, err := io.CopyN(blockdev.NewRWS(i.systemPartitionA), systemImage, i.SystemImage.Size()); err != nil {
 		systemImage.Close()
 		return nil, fmt.Errorf("failed to write system partition A: %w", err)
 	}